The use of coal as a fuel in electric generator plants is well known. In recent times, emphasis upon the burning of coal for this purpose has been increasing due to the dependence of this country on foreign sources for oil, the relative scarcity of oil, and the high price thereof.
As the emphasis has shifted to the use of coal fuel, considerable opposition to its use, especially to the use of "high sulfur" content fuel has steadily increased. Opponents of the use of coal rely upon the deleterious effects that SO.sub.x emission (resulting from the combustion of the coal) into the atmosphere have on the environment.
Generally, two approaches have been used to counter the problem of excess SO.sub.x emissions. The first has been to provide elaborate off-gas scrubbing units in combination with the coal boiler or furnace in coal combustion systems, usually in pulverized coal combustion systems. The more generally accepted systems utilize lime or limestone as preferred sorbents. These devices wash the gas and absorb the sulfur oxides, producing a slurry or sludge that contains the sulfur oxide in the form of calcium sulfite and calcium sulfate. Some of the sorbent material that is currently in use contains other ingredients such as dolomitic lime and/or proprietary additives. Generally, the commercial applications are based on the use of either finely ground calcium carbonate (limestone) or a suspension of calcium hydroxide which is produced from high calcium quick lime.
Commonly, the waste sludges produced by the aforementioned scrubbers contain a high proportion of relatively inert fine particulate matter and a variety of reactive materials in sufficiently small concentrations as to make the reclamation thereof uneconomical. These reactive compounds in the sludge solids include common materials such as sulfate ions, calcium ions, aluminum ions, and iron ions or sources thereof. The water content of such sludges may vary considerably and is typically in the range of 80-90% by weight.
The disposition of such sludges has always been a problem. With the increasing awareness of the undesirability of discharging such waste material into the environment and the resulting government regulations of such discharges, the need for a convenient, harmless and economical way of disposing of such sludges or converting them into a useful product is becoming increasingly apparent. A further incentive is provided by the capital investment and land required for the disposition of such sludges by conventional means, which often include large settling ponds or storage piles.
In most cases the sludge, prior to any chemical modification for disposal purposes, is subjected to a dewatering process whereby the liquid content of the sludge is decreased by means of a centrifuge and/or vacuum filter. Heretofore, sludge with high sulfite content has been extremely difficult to dewater to a commercially acceptable extent. In this regard, it has been common commercial practice to add various compounds to the sludge to effect oxidation of the sulfite to sulfate, which latter radical lends itself to better dewatering. Also, other attempts to dewater high sulfite content sludge have included the addition of certain polymeric substances to the sludge.
Another solution to the problem of excess SO.sub.x emission in coal combustion systems has been to provide a fluidized combustion bed of lime or limestone in which the coal is ignited. Preferably, the coal is pulverized and also floats within this fluidized bed during the combustion thereof. The suspended lime or limestone particles thus absorb substantial amounts of the oxidized sulfur content of the coal in the form of anhydrous sulfates. After the spent fluidized bed combustion residue has been removed from the coal boiler or furnace, problems with respect to the disposal thereof have been presented due to the above-mentioned governmental regulations pertaining to protection of the environment, and also due to the fact that the spent residue has heretofore been thought useful for few practical applications.
In my paper entitled "Development of Potential Uses for the Residue from Fluidized Bed Combustion Process," FE 2549-3, I have suggested that fluidized bed combustion residue be used (FBCR), inter alia, in the production of road base material, as a sorbent in gas scrubbers, and as a means for stabilizing scrubber sludge when fly ash is also added to a combination of the sludge-residue.
In my paper entitled "Development of Potential Uses for the Residue from Fluidized Bed Combustion Processes", FE 2549-6, I have suggested, inter alia, that a wet "blunging" pretreatment of the fluidized bed combustion residue is useful as a means of liberating the lime component of the residue and that the compressive strength of scrubber sludge may be increased by mixing of the residue, sludge, and fly ash in certain proportions.
The use of FBCR sorbents and FBCR-sludge mixtures has not been without problems, however. For instance, when the entire FBCR fraction is charged into a scrubber as a sorbent, the solids content of the sorbent is dramatically increased, causing mechanical problems due to the relatively large particle size of various components of the FBCR. Also, when an entire blunged FBCR fraction is used in a sludge mixture, dewatering problems have arisen due to the large available lime content of the residue, which lime slakes and is difficult to settle.
Accordingly, despite the foregoing, there remains a need for a method in which scrubber sludges can be converted into stabilized structural material for use as reclamation material in a commercially acceptable and economic fashion.
Also, there remains a need for a method in which the efficiency of conventional sludge dewatering techniques is improved.
Further, despite the foregoing, there remains a specific need for a method for utilizing fluidized bed combustion residue in an environmentally acceptable manner.
Of further importance is the need to provide a relatively inexpensive source of sorbent material in SO.sub.x gas scrubbers, due to the ever increasing costs of lime and limestone, which are conventionally used as SO.sub.x sorbent.
These and other objects are met by the present invention. In accordance with the invention, the precipitate from an agitated slurry of fluidized bed combustion residue is added to waste scrubber sludge in order to provide a novel composition useful in reclamation projects. If added prior to sludge dewatering, the residue precipitate acts as a filter aid and also undergoes certain hydration reactions to chemicaly bond water from the sludge, thus increasing the solids content of the sludge. If desired, the supernatant liquor separated from the fluidized bed combustion residue slurry may be fed into a SO.sub.x scrubber as a sorbent to absorb substantial amounts of SO.sub.x wherein x is 2 or 3 in the waste gas. This latter facet of the invention provides distinct economic advantage as it provides a relatively inexpensive source of lime for use as a sorbent.